The importance of Lyme disease as an emerging public health problem is well known. The disease can be acute or chronic in nature and the spectrum of disease manifestations elicited in this infection continues to expand. As Lyme disease has attained public and medical attention, the difficulties in the clinical diagnosis and the limitations of the currently available laboratory diagnostic tests have become apparent. Presently three genospecies of B. burgdorferi are considered to be responsible for Lyme disease. However, differences have been observed in the clinical manifestations of Lyme disease and there appears to be some correlation of clinical manifestations with some of the genospecies. The disease manifestations, whether they be acute or chronic, are associated with the presence of live spirochetes although they are few in number. Little is known about the virulence factors of these spirochetes nor their mechanism of pathogenicity. Additional methods for the characterization and identification of the various isolates are needed to better understand the pathogenesis and the epidemiology of this emerging disease. The overall objective of the proposed studies is the identification of virulence factors in Borrelia burgdorferi: I. Identify plasmid encoded genes associated with virulence and elucidate their function in B.burgdorferi A. Examine nucleotide sequences by transcription analysis and DNA sequencing. B. Analyze proteins expressed differentially in virulent clones which possess the plasmid lpB31.24 and avirulent clones lacking lpB31.24. C. Elucidate the function of gene products associated with lpB31.24 and cpB31.9. II. Molecular regulation of virulence factors in B. burgdorferi A. Characterization of two-component signal transduction proteins. B. Role of iron in expression of virulence factors in B. burgdorferi.